This invention relates to an improvement of a hydraulic control system, and more particularly, to a closed loop control method for the hydraulic control system.
The closed loop control of a hydraulic apparatus is generally performed by a method comprising the steps of detecting a speed and/or a pressure of an actuator disposed in a hydraulic cylinder means by a detector, comparing the value thus detected with a desired value of the speed and/or pressure preliminarily obtained, and controlling an input current for a drive mechanism such as servo-valve which controls the operation of the actuator in accordance with the compared result, i.e. the difference value therebetween. According to the closed loop control method of the type described above, as shown in FIG. 1, the relationship between the detected value and the desired value which was preliminarily set is 1:1 (i.e. is expressed linearly by an equation Y=X, line 1). This fact means that the actuator can be operated repeatedly under the same operational condition.
Although the hydraulic apparatus can be controlled by the closed loop control method as referred to above, the apparatus can also be operated under an opened loop control, in which a preliminarily set desired value regarding the speed and/or pressure of the actuator acts on a drive mechanism such as a servo-valve or proportional valve thereby to control an input current for the drive mechanism. However, the opened loop control is generally affected largely by an external disturbance such as a temperature, load pressure and the like which affect the operation of the actuator. In addition, the relationship between the desired value and the detected value regarding the speed and/or pressure of the actuator is expressed by a curve 2 as shown in FIG. 1. Moreover, in the opened loop control, it is difficult to maintain the operating condition, even if once determined, because of the external disturbance.
The operational characteristics or phenomenon described above will be described in detail hereunder in conjunction with FIG. 2, in which are shown relationships with respect to the closed and opened loop controls respectively between a desired value A and a value B regarding the quantity of electricity to be applied to a valve such as servo-valve of the hydraulic apparatus, between the value B and a value C regarding an opening degree of the valve, and between the opening degree C and an effective value D.
With the opened loop control, a value b regarding the quantity of electricity with respect to a desired value a (i.e. set value) is in the relationship 1:1, which is expressed by the equation B=A . . . (11) as shown in FIG. 2. The value b and a value c regarding opening degree of the valve are also in the relationship 1:1, which is expressed by the equation C=B . . . (12).
However, the relationship between the value c and an effective value d does not become a relationship 1:1 because of the external disturbance such as load pressure, temperature or the like affecting the actuator and is expressed by a non-linear function f(x), i.e. the equation D=f(c) . . . (13), which is predetermined with respect to the valve of the hydraulic apparatus to be used. According to the relationship A=B=C, the effective value D is represented by the following equation (14) in view of the relationship between the values A and B. EQU B=f(a) (14)
Therefore, the effective value with respect to the desired value a in the opened loop control does not become a value d and takes a value d'=f(c)=f(a).
On the other hand, with the closed loop control, the relationships between the desired value A and the value B regarding the quantity of electricity, between the value B and the value C regarding the opening degree of the valve, and between the value C and the effective value D are all in the relationship 1:1 as shown by the lines 11, 12 and 13' as shown in FIG. 2, thus the effective value being a value d with respect to the desired value a.
With the closed loop control, as is apparent from the aforementioned reasons, defects or disadvantages of the closed loop control, against its easy operation, will be recognized as follows.
(a) In a case where two hydraulic machines or apparatus which are provided with the closed and opened loop control functions attain the same controlling ability, respectively, the operating conditions of the machines or apparatus are different from each other in spite of the same controlling ability. In other words, as can be understood from FIGS. 1 and 2, the detected value y in the closed loop control is larger than that in the opened loop control by a value .DELTA.y with respect to the same desired value x in the both controls. This fact means that in the closed loop control of the hydraulic apparatus are detected the speed and pressure higher than those in the opened loop control. Thus, even if the desired valves were preliminarily set to the same values for the hydraulic machines or apparatus provided with the closed and opened loop control functions, respectively, the detected speed and pressure are different with respect to the both hydraulic machines, which requires a troublesome procedure for controlling or handling the machines.
(b) Regarding resolving powers of the controls, as can be understood from FIG. 1 in which when the desired value x.sub.1 is changed to the value x.sub.2 (x.sub.1 &lt;x.sub.2), the variation of the resolving powers of the opened loop control is .DELTA.y.sub.1 and that of the closed loop control is .DELTA.y.sub.2 which is larger than .DELTA.y.sub.1, the opened loop control is superior to the closed loop control in a low speed operation region of the hydraulic machine. Accordingly, in order to identify the resolving power of the closed loop control to that of the opened loop control, it is necessary to finely divide the desired value in the closed loop control to obtain substantially the same resolving power as that in the opened loop control.
(c) Since, in a hydraulic machine of the closed loop control, the control in the low speed operation region is considerably difficult, the closed loop control is not generally suitable for molding a product which is relatively thick, which requires a low molding speed control, for example, for a lens production in an injection molding machine.